1. Field of the Invention
The present invention relates to a direct fuel injection-type spark-ignition internal combustion engine.
2. Description of the Related Art
There has heretofore been known stratified charge combustion produced by directly injecting fuel into a cylinder to form a mixture that can be favorably ignited (combustible mixture) only in the vicinity of a spark plug, at the ignition timing, to burn a lean mixture in the cylinder as a whole. To carry out the stratified charge combustion, in general, fuel is injected in the latter half of the compression stroke. It is intended that the thus injected fuel proceeds into a concave combustion chamber, is vaporized by robbing heat from the wall surfaces of the combustion chamber, is deflected by the shape of the combustion chamber toward the spark plug, and forms a combustible mixture near the spark plug.
Japanese Unexamined Patent Publication (Kokai) Mo. 9-158736 proposes injecting the fuel in the shape of a flat fan having a relatively small thickness by using a fuel injection valve having an injection hole in the shape of a slit. In comparison with general fuel spray having a cone shape, the thus injected fuel can rob heat from a wide area of the wall surfaces of the combustion chamber, making it possible to form a combustible mixture within a short period and to retard the timing for ending the injection of fuel. Thus, it is possible to increase an amount of injected fuel in the latter half of the compression stroke and to expand the region of stratified charge combustion toward the high engine load side.
After impinging the bottom wall of the combustion chamber, the fuel injected in a flat fan shape spreads in the width direction while progressing along the bottom wall of the combustion chamber. Therefore, the fuel can favorably vaporize by robbing heat from a wide area of the bottom wall. However, on the other hand, to form a mass of combustible mixture near the spark plug, each part of the fuel spreaded in the width direction must be directed near to the spark plug by the side wall facing the fuel injection valve in the combustion chamber. The side wall has a circular arc shape in plan view. Therefore, when the spread fuel moves up along the side wall of the combustion chamber after progressing along the bottom wall, each part of fuel is given a speed component upward and a speed component toward the center of the wide direction of the fuel spray. Thus, all of the fuel moves near to the spark plug arranged above the center of the width direction of the fuel spray and a mass of combustible mixture is formed in the vicinity of the spark plug.
In general, the spark plug is arranged above the center of the width direction of the fuel spray and inside of the side wall, i.e., above the circumference portion of the combustion chamber. Accordingly, to direct each part of the fuel to the vicinity of the spark plug, the side wall has a deflection portion inclined to the inside of the combustion chamber, and the deflection portion and the bottom wall of the combustion chamber are smoothly connected by a circular arc portion. The angle of the deflection portion with the bottom wall of the combustion chamber and a radius of the circular arc portion are important factors to surely direct each part of the fuel to the vicinity of the spark plug. If these are inappropriately selected, a mass of combustible mixture may not be formed in the vicinity of the spark plug and thus good stratified charge combustion may not be realized.